Factors Controlling Resilience and Resistance of Coastal Salt Marshes to Sudden Marsh Dieback

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Sudden Marsh Dieback - SMD - has been documented for the past two decades throughout coastal areas of the United States. With these large-scale diebacks comes the loss of ecosystem functions and services. USGS scientsts use field work and greenhouse studies to investigate the factors that control the resilience and resistance of coastal salt marshes to SMD. 

Dieback along the Northern Gulf of Mexico

Dieback along the Northern Gulf of Mexico 

Science Issue and Relevance: Coastal wetlands provide numerous critical ecosystem services such as storm surge protection, fisheries production, improvement of water quality, and carbon sequestration. Within the past two decades, large-scale sudden dieback of salt marshes, termed Sudden Marsh Dieback (SMD), has been documented throughout the coastal United States, from Louisiana to Maine, resulting in the acute loss of ecosystem functions and services. The largest dieback events recorded in the southeastern U.S. and Northern Gulf of Mexico caused complete mortality of over 100,000 hectares of Spartina alterniflora-dominated salt marsh. Drought was identified as a major ecosystem driver of these large-scale disturbances. The 2000 dieback along the Northern Gulf of Mexico coincided with the most intense drought recorded in the previous 100 years. Similarly, the 2002 dieback in the southeastern U.S. followed the driest three-year period on record. However, in 2009, and most recently in 2011 - 2012, sudden marsh dieback was observed following periods of elevated water levels, indicating that extreme conditions on either end of the hydrological spectrum may result in sudden marsh dieback.

In addition to the loss of the vegetated community and critical ecological functions, recovery of these degraded systems is made even more difficult by elevation collapse following plant mortality, which results in excessive flooding and suboptimal growing conditions. Thus, one method of restoration explored the beneficial use of dredged sediment in enhancing marsh elevation. Following the 2000 dieback, large-scale experimental manipulations were conducted to determine how additions of dredged sediment-slurries would impact the degraded dieback marshes. Researchers found that immediately following sediment addition (< 5 years), the restored sites at higher elevations had greater productivity compared to the natural marshes. Additionally, twelve years later in 2012, preliminary observations indicated that areas restored with sediment addition at higher elevations were resistant to the most recent dieback event; however, in contrast, the low-lying natural marshes appear to have been more vulnerable to the recent dieback event (Figure 1).

Controlled greenhouse experiments manipulating the marsh elevation and flood duration

Controlled greenhouse experiments manipulating the marsh elevation and flood duration

Methodologies for Addressing the Issue: We investigated the role of elevation capital in controlling the ecological resistance of restored salt marshes to climate change-induced disturbances. We predicted that wetlands with greater elevation capital were more resistant to disturbance, and that sediment addition can restore a wetland to an optimal elevation, where ecological resistance is equivalent to or greater than natural marshes. The existing dieback pattern observed at the sediment-restored research site (Figure 1) provided an excellent opportunity to test these hypotheses. The restored site contained four sediment- amended marshes, each containing four levels of sediment addition, or four different elevations. Reference sites, natural marshes that did not receive sediment additions, were also included in the experimental design. In addition, controlled greenhouse experiments manipulating the marsh elevation and flood duration at multiple levels of atmospheric CO2 were conducted to investigate the interactive effects of multiple stressors on S. alterniflora physiology and resistance to disturbance. S. alterniflora were exposed to a combination of elevated (720 ppm) or ambient (400 ppm) atmospheric CO2 and one of three levels of flood treatments at multiple marsh elevations. Continuous flooding treatments were used to mimic water-logged conditions that can result from relative sea-level rise, subsidence and altered hydrology. Drought treatments were implemented to mimic severe drought conditions that preceded the 2000 SMD event. Spartina alterniflora response to the excessive flood or drought treatments at multiple elevations was compared to S. alterniflora grown in normal tidal conditions to determine which stressor combination is capable of inducing dieback and how climate change-induced elevated CO2 may affect resistance to disturbance.

Future Steps: Future studies will examine the long-term recovery of the restored and natural salt marshes after multiple dieback events in an effort to isolate conditions that promote resistance or enhance vulnerability. Future greenhouse studies will be conducted to analyze the interactive effects of flooding and CO2 on fungal pathogens known to be associated with dieback impacted salt marshes.